Method of and means for reducing retroactive currents in audion amplifiers



Dec. 6, 93 s. BALLANTINE METHOD OF AND MEANS FOR REDUCING RETROACTIVE CURRENTS IN AUDION AMPLIFIERS Filed July 14, 1925 Patented Dec. 6, 1932 UNITED STATES T OF C' STUART BALLANTINE, or W'HITE HAVEN, PENNSYLVANIA, Ss eNoa, BY MESNE ASSIGNMENTS, To iaAnIo CORPORATION on AMERICA, on NEW YORK,I1\T. Y., A con IPORATION or ELAW RE METHOD OF AND MEANS FOR REDUCING RETROACTIVE GURRENTS IN AUDION' AMPLIFIERS Application filed July 14, 1925. Serial No. 43,635.

This application is a continuationin part of my application Ser. No. 629,702, filed I April 3, 1923. The present invention relates to a method of compensating an audion amplifier, or a cascaded seriesof such amplifiers, for the retroactive currents which fiow through the inherent grid-plate capacity of an audion tube. By this compensation I diminish or completely eliminate the disturbing effects which commonly arise from the fact that variations in potentialacross the plate circuit impedance affect the input circuit terminals of an audion amplifier stage thlrjough the grid-plate capacity of theaudion tu e.

In carrying this invention into effect I arrange the impedance network of each amplifying stage in the form of an alternating current IVheatstone bridge circuit'in which the input circuit of the stage ancl'output circuit of the stage (which may coincide with the output circuit of the tube) form conjugate arms of the bridge. In particular, I form such a bridge circuit having four capacitive arms one of which is thegrid plate capacity of the amplifier tube. By an alternating current Wheatstone bridge I mean six impedances arranged in trilateral symmetry about a point. Two of these impedances I term conjugate arms of the bridge, and the remaining four I term balancing arms of the bridge.

Referring now to the drawing:

Fig. 1 is an amplifier compensated accord ing to my invention;

Fig. 2 is a diagrammatic representation of the essential alternating current elements of the circuit of Fig. 1 as arranged to form a conventional bridge diagram;

Fig. 3 is a cascaded amplifier employing the circuit of Fig. 1.

In Fig. 1, C represented by the dotted lines, is theinherent grid-plate capacity of the amplifier tube. C represented by the dotted lines, is the inherent grid-filament capacity of the tube. I may or may not employ an additional external capacity be tween the grid and filament. In order to render my figure as general as possible I have shown such an external capacity at C C and C are connected in parallel, their valuesare therefore additive, and their sum is C C is always equal to C plus C but as stated above, the value of C may bezero.

When C is omittedfthecapacity C has a magnitude larger than capacity C C and C are two capacities connected in series in the plate circuit of the tube. They are shunted by a coil L which serves as a path for the direct current fiowing in the plate circuit and thereby allows the B-battery potential to be impressed upon the plate. It may also serve as a couplingicoil whereby the plate circuit of one amplifying stage is coupled to the input circuit of the. next succeeding stage; Any suitable coupling dement might be used in place of this coil,.as I

for example, a resistance. I

Theinput-circuit of the stage is connected betweenth e-gridand the junction of the sec- .Wheatstone bridge having four capacitive arms between opposite junction points of which are connected conjugate arms ofthe bridge which comprise the input and output circuits of the amplifier stage. Since the four spaced points G, F, 2 and P are interconnected by a bridge network, it is obvious that the relative values of the four bridge arms may be so proportioned as to electrically isolate the conjugate arms G2 and PF from one another. I Since the coil L is connected across a conjugate arm of the-bridge, and not across one of the four balancing arms, it does not aifect the balance of the bridge and need not be taken into account in the diagram of Fig. 2. This is also true of the plate-filament capacity of the tube. v ,7 I

The condition for balance of this bridge is that as may be seen from Fig. 2. So long as this condition is fulfilled any or all of the capaci ties may be varied, as for example, for tuning purposes. In practice the value of C is fixed by the tube employed. This leaves C C, and C which may be varied in any way so long as the balance condition be fulfilled. Thus, 0;, and C may be simultaneously varied, their ratio being kept constant; or C and 0,, may be varied in a constant ratio. In general, any combination may be varied so long as the condition for balance is maintained. Such variations do not interfere with the compensating action ofmy bridge circuit.

simplicity, as have also the filament batteries,

rheostats, and other usual partsof the circuits. Since the B-battery is commonly bypassed, or so designed that its alternating current resistance is negligible, it is immaterial whether the'connection from C to the filament is made on the positive or negative side of the B-battery. A common B-battery may of'course be used but has been omitted from the drawing for the sake of simplicity.

So also a high resistance (grid-leak) may be interposed between the grid and the filament circuit.

It will be seen that the bridge balance is theoretically independent of frequency. This is because the four balancing arms of the bridge are each substantially pure capacity, and not composite arms. In practice I obtain results which closely approximate this theoretical independence of frequency, and by a single adjustment may compensate for a long range of wave lengths.

It is obvious that the balance may be complete or not as I desire, and hence that the compensation may either completely eliminate or merely diminish the effects of the flow of retroactive currents. i

It should be understood that when I refer to a capacity between the grid and filament of an audion tube I may mean either the inherent grid-filament tube capacity or this capacity in parallel'withan' external on e. No departure from the spirit of my inven tion is involved in the. fact that'the several capacitive arms of the bridge are physical condensers which may have small alternating current resistances. I am fully aware of the several physical reasons, including the flow of space currents within the tube, which may prevent :these capacitive arms from acting as mathematically pure capacities and, therefore, when I state that the balancing arms of my bridge are pure capacities, I mean that they are substantially pure capacities, the absolute valuesof which do not materially vary with frequency.

It should be further understood that the compensation obtained by the lVheatstone bridge arrangement is due to the cooperation of-the four capacitive arms of the Wheatstone bridge and that no one. of these arms is more important in accomplishing the compensation than is any other arm.

I claim:

1. In a method of preventing potential variations across the plate circuitimpedance of an audion amplifier stage from affecting the input circuit of said stage, the step which comprises arranging the impedance network of said amplifier stage in the form of a VVheatstone bridge having four capacitive arms one of which is the capacity between the grid and plate of the audion tube.

2. In a method of compensating for the flow of retroactive currents through the gridplate capacity of an audionamplifier tube, the steps which comprise arranging the impedance network of the amplifier stage in the form of a'four capacitive arm Wheatstone bridge, and additionally arranging the input circuit of the tube as one conjugate arm and theoutput circuit of the tube as another con jugate arm.

3. In an audion amplifier, an audion tube having an inherent grid-plate capacity, a capacity between the grid and filament of said tube, two capacities in the plate circuit of said tube, and circuit connections whereby the said capacities are connected to form a vVheatstone bridge having four capacitive arms.

4. Apparatus for amplifying electric oscillations comprising a plurality of audion amplifier stages, each of said stages including an impedance network arranged as a Wheatstone bridge having four capacitive arms,

and havin as con'u ate arms the in ut and the output circuits, respectively, of the stage; the output circuit of one stage being coupled to the input circuit of the succeeding sta e.

The method oftuning an audion amplifier stage the impedance network of which is arranged to form a lVheatstone bridge circuit having four capacitive arms one of which is the capacity between the grid and plate of the audion tube, which comprises varying the capacities forming the arms of said Wheatstone bridge whilemaintaining between said capacities such a relation that said bridge remains substantially in balance.

6. The method of tuning an audion amplifier stage the impedance network of which is arranged to form a WVheatstone bridge having four capacitive arms one of which is the grid-plate capacity of the audion tube, which comprises simultaneously varying two of the capacities forming the arms of said bridge while maintaining between said capacities a constant ratio such that said bridge remains substantially balanced.

7 An impedance network for an audion amplifier stage comprising a capacity be tween grid and plate of an audion tube, a capacity between grid and filament of said tube, two capacities connected in series between plate and filament of said tube, and a connection from one input terminal of said stage to a point intermediate said two capacities between plate and filament.

8. An audion amplifier stage the impedance network of which comprises an alternating current Wheatstone bridge having two conjugate arms electrically isolated from one another, and forming respectively the input and output circuits of the amplifier stage, and four balancing arms comprising substantially pure capacities, one of said balancing arms comprising the capacity between grid and plate of the audion tube.

9. A radio receiving apparatus comprising a repeating coil having a secondary and a vacuum tube having a filament, a grid and a plate, elements forming in efiect a Wheatstone bridge arrangement, substantially pure capacity only forming each arm of said bridge, the secondary of said repeatingcoil being connected between opposite junctions of said bridge, one such capacity being formed by the grid to plate capacity of said tugs, to thereby suppress oscillation of the tu e.

10. An audion amplifier stage the impedance network of which comprises an alternating current VVheatstone bridge having one conjugate arm comprising the input circuit of the amplifier stage, and four balancing arms each comprising substantially pure capacity, one of said balancing arms comprising the capacity between grid and plate of the audion tube.

11. An amplifier stage wherein a vacuum tube having grid, plate and filament, and a plurality of impedances form an alternating current Wheatstone bridge, characterized by the fact that the four balancing arms each comprise substantially ure capacities, two of which are located between grid and filament,

and between grid and plate, respectively, and

two of which are serially connected between plate and filament, said grid and the junction of said two serially connected capacities serving as terminals for the input circuit of said stage, and said plate and filament serving as terminals for the output circuit thereof.

12. The combination with a three-electrode thermionic device having an input circuit, a capacity connected between a point in said input circuit and the cathode of said thermionic device and having a magnitude substantially greater than the magnitude of the capacity between the grid and cathode of said thermionic device to cause between said point and the cathode a substantial difference of potential, a second capacity connected between said point and a source of disturbing potential, and an input element having-its terminals connected, respectively, to said point and to the grid of said thermionic device. I 13. The combination with a-three-electrode valve of a divided capacity having a lead therefrom connected at the midpoint to the cathode and an end connected to one of the other electrodes and a balancing capacity connected betweenthe other electrode and the other end of the capacity.

14. Radio receiving apparatus comprising a three-electrode thermionic device comprising an anode, cathode'and control electrode, an input element connected to said control electrode and to a branch point, a capacity in a connection from said point to said anode,

ent grid-filament capacity, and a pure capacity bridge comprised of aplurality of capac- I.

ities associated with the input circuit, said bridge being elfective to neutralize without inductive reactionssaid inherent capacitive coupling, the capacities of said bridge being so proportioned that the potential of the grid is substantially unaffected by changes of platepotential, at least one of the capacities of said bridge being connected between the grid and filament electrodes of said audion and being of a value substantially larger than that of said inherent grid filament capacity.

In testimony whereof, I affix my signature.

STUART BALLANTINE. 

